Tiny Plants That Once Ruled the Seas


By Ronald Martin /Antonietta Quigg R. マーティン /A. クイッグ
English 日本語 日本語
If you could hop onboard a time machine and visit the earth as it was 500 million years ago, during the Paleozoic era, you’d be forgiven for thinking you had traveled not to another time period but to another planet altogether. In essence, you would have. The continents mostly sat in the Southern Hemisphere, the oceans had vastly different configurations and currents, the Alps and the Sahara had yet to form. Land plants had not even evolved. Perhaps the most dramatic difference, however, would lie in the animals that inhabited this primeval earth. Back then, most of the world’s multicellular creatures lived in the sea. Clamlike creatures called brachiopods and trilobites—those extinct cousins of today’s lobsters and insects, with their hard exoskeletons, long antennae and compound eyes—reigned supreme.  タイムマシンに乗って5億年前の古生代の地球を訪れたとしよう。おそらくあなたは,別の時代にタイムスリップしたのではなく,知らない惑星にたどり着いたと考えるだろう。無理もない。そこは実質的に別の惑星だ。大陸のほとんどが南半球に存在し,海の形状も海流も現在とは大きく異なり,アルプス山脈もサハラ砂漠もまだ形成されていなかった。陸上植物はまだ現れていない。
The diversity of marine animals grew substantially over the next 250 million years, until the so-called Permian extinction event snuffed out more than 90 percent of ocean species and brought the Paleozoic to a close. The loss of life was staggering. But change was on the horizon, and while life on land underwent a radical transformation with the rise of dinosaurs and mammals, life in the sea entered a dramatic phase of reorganization that would establish the dominance of many of the animal groups that prevail in the marine realm today, including modern groups of predatory fish, mollusks, crustaceans, sea urchins and sand dollars, among others.  その後の2億5000万年の間に海洋動物はかなり多様化した。だが,いわゆるペルム紀末の大絶滅で海洋無脊椎動物種の90%以上が姿を消し,古生代が終わった。生物の減少は想像を絶するものだったが,変化の兆しは見えていた。陸上生物は恐竜と哺乳動物の登場に伴って様変わりする一方,海洋生物は大規模な再編成の時期に突入し,捕食魚や軟体動物,甲殻類,ウニなど,今日の海洋域で見られる多くの新しい動物群による支配が確立された。
The fossil record shows that over the ensuing Mesozoic and Cenozoic eras, marine life diversified at an unprecedented rate—so much so that scientists once questioned whether the pattern merely reflected the preferential preservation of geologically younger fossils, which have had less time to undergo erosion. Subsequent analyses indicated that this apparent florescence of species in the sea was indeed real, however. To explain the phenomenon, researchers have turned to a range of factors, including changes in climate and sea level, as well as mass extinctions, all of which could have fostered new opportunities. Yet although all these events may have contributed to the diversification that began around 250 million years ago, they cannot alone account for the pattern of the observed explosion.  その後に続く中生代と新生代には,海洋生物が前例のない速さで多様化したことを化石記録が物語っている。あまりに大きな多様化なので,侵食を受けた時間の短い,新しい時代の化石が多く残っているだけではないかと疑われたほどだ。しかし,その後の解析で,海洋生物種のこの繁栄が見かけのものではなく,実際に起こっていたことがわかった。
There is another, underappreciated factor to consider: food availability. It turns out that increases in the quantity and nutrient content of microscopic plants known as phytoplankton, which form the base of ma­­rine food pyramids, accompanied the stunning emergence of new sea creatures in the Mesozoic and Cenozoic. We submit that the evolution of these modest plants fueled the rise of the modern marine fauna. This novel understanding of how phytoplankton transformed life in the ancient seas has something to say about the future of our planet as well. Phytoplankton continue to support food pyramids today; however, if future climate change and deforestation disrupt controls on their proliferation, as they have already begun to do, these plants could become a force of destruction.  これまで正当に評価・考慮されてこなかった別の要素がある。利用可能な食物の量だ。海洋の食物連鎖(生態系ピラミッド)を底辺で支える植物プランクトンの量と栄養分の増加が,中生代と新生代における新種の海洋生物の大量出現と同時に起こっていたことが判明した。こうした地味な植物プランクトンの進化が,現代の海洋動物相の出現を促したと私たちは考えている。